Anatomy of a mixed-influence shelf-edge delta, Karoo Basin, South Africa

摘要

The position and process regime of paralic systems relative to the shelf edge rollover is a major control on sediment transfer into deep water. The depositional strike and dip variability of an exhumed Permian shelf edge succession has been studied in the Paardeberg Ridge, Karoo Basin. Siltstone-rich slope turbidites are overlain by 25–75 m-thick prodelta parasequences. These are truncated by a 30 m-thick sandstone-prone unit of tabular or convex-topped sandstones, interpreted as wave-modified mouth bars, cut by multiple irregular concave-upwards erosive surfaces overlain by sandstones, interpreted as distributary channels. The stratigraphic context, lithofacies and architecture are consistent with a mixed-influence shelf edge delta; the erosional base to the unit marks a basinwards shift in facies, consistent with a sequence boundary. Channels become thicker, wider, more erosive and incise into deeper-water facies downdip and correlate with sandstone-rich upper slope turbidites, all of which support the bypass of sand across the rollover. The overall progradational stacking pattern results in a stratigraphic decrease in channel dimensions. The results of this study suggest a predictable relationship between channel geometry, facies and position on the shelf-to-slope profile under a mixed wave and fluvial process regime.